Inside a protective enclosure which provides containment and isolation of parts of the reactor containing material which is liable to be radio-active to some extent, pressurized water nuclear reactors have a vessel which contains the core of the reactor, a primary circuit in communication with the reactor core, in which pressurized water circulates, and at least two steam generators, each disposed in a loop of the primary circuit.
The enclosure for protection and containment of the reactor is constituted by a very large concrete building which contains in particular the steel vessel of the reactor inside which the fuel assemblies constituting the core of the nuclear reactor are arranged, in which the release of heat is produced during fission of the fuel. The vessel of the nuclear reactor is filled with pressurized water and connected to the primary circuit inside which this water, which is heated by contact with the reactor core, circulates.
The primary circuit is constituted by at least two (but generally three or four) distinct loops in communication with the interior of the reactor vessel. Disposed in each of the loops of the primary circuit, there is a primary pump assuring circulation of the pressurized water and a steam generator assuring the exchange of heat between the primary pressurized water bringing the heat from the core and the feed water the vaporization which is assured to supply a turbine associated with the reactor.
Pressurized water circulates in the steam generator at its primary side, i.e., inside tubes of small diameter and great length arranged in large numbers inside the casing of the steam generator.
The feed water circulates at the secondary side of the steam generator, i.e., in its casing and outside the small-diameter tubes.
When the nuclear reactor is in operation, the pressurized water is at a temperature close to 300.degree. C. and at a pressure of the order of 155 bars.
However, the nuclear reactor does not operate in an entirely continuous way, and it is sometimes necessary to shut it down, this being done by introducing control rods made of neutron-absorbing material into the reactor core, to maximum insertion position.
The water of the primary circuit is, however, still at a high temperature and at high pressure and, depending on the type of reactor shutdown required, it is necessary to achieve more or less complete cooling and depressurization of the water of the primary circuit of the reactor.
In particular, when shutdown must be effected for maintenance purposes, it is necessary to lower the temperature and pressure of the reactor to a very low level and at any rate to less than 70.degree. C. as regards temperature and less than 38 bars as regards pressure.
In the case of shutdown for the purpose of reloading, it is in fact necessary to lower the pressure to the value of the atmospheric pressure, and the temperature in the final stage must be between 10.degree. and 60.degree. C.
In the case of reloading, it is necessary in fact to open the cover of the reactor vessel so as to have access to the fuel assemblies constituting the core.
These two types of reactor shutdown, with lowering of the temperature and pressure of the primary fluid to a very low level, are termed cold shutdowns of the reactor.
Shutdowns are generally called cold shutdowns when the temperature in the final stage is less than 90.degree. C. and the pressure less than 28 bars.
In all other cases, shutdowns are called intermediate shutdowns or hot shutdowns; these allow the reactor to be rapidly restarted at its nominal operating conditions.
When cooling of the primary fluid is required, during a cold shutdown, the cooling capacity of the steam generators is used in a first stage and these are fed with secondary water by the stand-by feed circuit, the steam produced being directed towards the condenser via the by-pass for the turbine or evacuated into the atmosphere.
The pressurized water circulating in the primary circuit can be cooled in this way to a temperature of close to 180.degree. C.
To continue cooling of the primary fluid, a special cooling circuit termed shutdown cooling circuit (RRA) is used.
This shutdown cooling circuit is connected in parallel with two loops of the primary circuit of the reactor and generally comprises two heat exchangers which effect cooling of the primary circuit water by the cooling water conveyed to these heat exchangers. The shutdown cooling circuit also includes two circulation pumps and a portion of circuit which allows the flow in the heat exchanger to be short-circuited and regulated.
The whole of the assembly constituting the RRA circuit can be incorporated in the reactor containment vessel or conversely can be disposed outside this enclosure.
When the assembly is incorporated in the enclosure, containment of the primary fluid can thus be assured in all types of operation.
This does, however, impose more rigorous conditions as regards maintenance and more exacting conditions as regards the design of the equipment.
When cooling circuit equipment is not incorporated in the containment enclosure, the design and maintenance of installations are simplified but a single containment of the primary fluid, i.e. radiological protection and isolation of possible leakages in all instances, is no longer obtained except by going to the expense of providing another containment holding the equipment not incorporated in the first.
In addition, the primary fluid which circulates in the cooling circuit is at a temperature which at the start is of the order of 180.degree. C. and at a pressure which is always greater than or equal to 28 bars. In practice, to keep the primary pumps operating, it is necessary to have a pressure at least equalling 28 bars, since the primary pumps are only able to operate at a pressure equal to or greater than this limit if the appearance of cavitation phenomena is to be avoided and proper functioning of the seals guaranteed.
In addition, the primary pumps must be kept operating to maintain proper chemical and thermal homogeneity of the primary fluid.
The primary fluid is therefore at a high temperature and pressurized when it flows into the cooling circuit. This makes it essential to provide for adequate safety conditions when designing and protecting the cooling circuit, particularly against internal and external missiles.
In addition, actually cooling the primary fluid with the cooling circuit exchangers creates thermal heterogeneities when a primary pump is no longer available. In this instance in particular, the temperature of the steam generators continues to be high.